1. Field of the Invention
This invention relates to an image-forming apparatus based on an electrophotographic system or an electrostatic recording system and, more particularly, to an improved transfer material carrying member to be used for transferring a toner image formed on an image carrier onto a transfer material in such an image-forming apparatus. For the purpose of the present invention, an image-forming apparatus may be an electrophotographic copying machine, a printer or some other image recording apparatus that may be used for producing black and white images, monochromatic images and/or color images.
2. Related Background Art
In an image-forming apparatus operating on the basis of an electrophotographic system or an electrostatic recording system, a toner image formed on an image carrier is transferred onto a transfer material to produce a final image.
For example, in an electrophotographic apparatus involving a series of image forming operations of electrification, exposure to light, development, image transfer and cleaning typically comprises a transfer drum and a transfer unit illustrated in FIGS. 4 and 5 of the accompanying drawings and denoted by reference numeral 10 and 10A, respectively, in order to transfer a toner image formed on a photosensitive member 33 that operates as an image carrier onto a transfer material such as a sheet of paper.
Referring to FIG. 4, the transfer drum 10 comprises a support body formed by a pair of cylinders 12 and 13 arranged at the opposite ends of the drum and a connecting member 14 for connecting the cylinders and provided with a transfer material carrying member 11 covering an opening on the outer periphery of the support body. The connecting member 14 is provided with a gripper 15 for gripping the transfer material fed from a transfer material feeding unit. Additionally, as shown in FIG. 5, there are provided a transferring discharger 21, an inside diselectrifying discharger 23 and outside diselectrifying dischargers 22 and 24.
A transfer material is fed to the transfer drum 10 in synchronism with the formation of a toner image on the photosensitive member 33 and held onto the transfer drum 10 as it is gripped by the gripper 15 at the leading edge. Then, it is repeatedly made to pass through a transfer section disposed vis-a-vis the photosensitive member 33 as the transfer drum 10 rotates in the sense indicated by arrow a. Each time the transfer material passes through the transfer section, it is subjected to a corona discharge of the transferring discharger 21 from the back side of the transfer material carrying member 11 with a polarity opposite to that of the toner in order to transfer the toner image on the photosensitive member 33 onto the transfer material.
After receiving a required number of times of toner image transferring operations, the transfer material is diselectrified by the diselectrifying dischargers 22, 23 and 24, peeled off from the transfer drum 10 by means of a separation claw 28 and then moved to a fixing unit.
In order to obtain a high quality image, the toner image on the photosensitive member has to be accurately transferred onto the transfer material. In other words, a high transfer efficiency is required to achieve an enhanced degree of image quality for transferred images.
In order to improve the transfer efficiency, the toner on the photosensitive member has to be firmly and strongly attracted to the transfer material. Therefore, it is important to raise the intensity of the electric field applied to the transfer material and the transfer material carrying member as much as possible. In order to obtain a uniformly transferred image, it is important to make the transfer material carrying member free from electric leakage.
The above requirements can be effectively met by using a transfer material carrying member having a low relative dielectric constant. However, the use of a transfer material carrying member having a low relative dielectric constant can in turn lead to a problem of producing images with a low image density particularly when a large number of images are transferred continuously.
The above problem can become particularly remarkable when the developer contains fine particles of toner with a diameter less than 10 .mu.m or an average particle diameter of about 8 .mu.m, which are often used in recent years to improve the reproducibility of electrostatic latent images and hence the quality of final visible images.